1. Field of the Invention
The present invention relates to a switching regulator for obtaining a stabilized DC output from an AC power source, in particular, to a control system of a power factor correction circuit for suppressing higher harmonic current flowing out to the AC power source side.
2. Description of the Related Art
In order to suppress higher harmonic current flowing out to the AC power source side, power factor correction circuits using a boosting chopper are widely applied to switching regulators, the boosting chopper comprising a rectifier bridge, an inductor, a switching element, a diode or a synchronous rectifier switching element, and a capacitor. Switching regulators in a several hundred watts class, in particular, generally uses a system to control the on-off timing of the switching element to operate in a continuous current mode in which the current running in the inductor is continually in the positive polarity. Control of a conventional power factor correction circuit in the continuous current mode operation has been conducted primarily in the average current control system and the peak current control system.
FIG. 4 shows a conventional control circuit in a current control system. In order to stabilize the DC output voltage Vo while controlling a current running in the side of the AC power source 1 to be a sinusoidal waveform in this current control system, a voltage error amplified signal that is a difference between the DC output voltage Vo and a command voltage Vref is generated by a voltage error amplifier 11 and a current command value Vi that is a product of the voltage error amplified signal and an output voltage Vd of the rectifier bridge is generated by a multiplier 12
A current error amplifier 17 generates a current error amplified signal that is a difference between an inductor current signal that is a signal of inductor current IL detected by a current detecting resistor 9 and the current command value Vi. A comparator 18 compares the magnitudes of the current error amplified signal and a carrier signal with a saw tooth or triangular waveform at a constant frequency that is generated and output by a carrier signal generating circuit 19, to generate a control signal Vg for giving to a gate of the switching element 6. Thus, on-off timing of the switching element 6 is controlled. A circuit for setting a feedback constant is to be connected between the output and input sides of each of the voltage error amplifier 11 and the current error amplifier 17 in FIG. 4. The circuit for setting a feedback constant, however, is omitted in FIG. 4.
Next, FIG. 5 shows a conventional control circuit in the peak current control system. A control circuit disclosed in Japanese Unexamined Patent Application Publication No. 2001-028877 is a known example of the peak current control system shown in FIG. 5. The peak current control system shown in FIG. 5 differs from the current control system shown in FIG. 4 in that the peak current control system of FIG. 5 turns the switching element 6 on by the output signal of the pulse generator circuit 20 at a constant frequency and turns the switching element 6 off by detecting, with the comparator 13, an event that the inductor current signal, a magnitude of the inductor current IL detected with the current detection resistor 9, has reached the current command value Vi, while the average current control system shown in FIG. 4 determines on-off timing of the switching element 6 by comparing the magnitude of the current error amplified signal with the magnitude of the output carrier signal of the carrier signal generator circuit 19. The peak current control system shown in FIG. 5 does not need a current error amplifier 17 shown in FIG. 4. The peak current control system in FIG. 5 is depicted omitting, like in FIG. 4, a circuit for setting a feedback constant to be connected to the voltage error amplifier 11.
The control circuit in the peak current control system as shown in FIG. 5 has a problem that an unstable oscillation phenomenon called sub-harmonic oscillation occurs when the output signal of the pulse generator circuit 20 is made at a constant frequency. The Publication of Japanese Unexamined Utility Model Application No. H05-009187 (FIG. 1) copes with this problem by slope compensation corresponding to the input voltage.
Japanese Unexamined Patent Application Publication No. H04-168975 and Japanese Patent Gazette No. H08-032182 (FIG. 1), (“'182”) discloses a control method for stable oscillation without the slope compensation by fixing the off time of the switching element. In this method, when the inductor current signal exceeds the current command value, a monostable multivibrator generates a minute off-time with a fixed period of time.
Japanese Unexamined Patent Application Publication No. H08-168248, (“'248”) discloses an AC-DC convertor in which the power factor is enhanced and, at the same time, the off-time of switching is made longer when the rectified voltage of the AC input voltage is large and made shorter when the rectified voltage is small. The off-time of switching in this AC-DC converter is varied at every moment in one cycle of the AC power source frequency, according to the rectified voltage measured on a real-time basis.
The conventional current control system as shown in FIG. 4 has problems that it requires two error amplifiers making the control circuit large sized and that it needs complicated adjustment of feedback constants.
The conventional peak current control system as shown in FIG. 5, although it requires one error amplifier maintaining the control circuit relatively small sized, has a problem that stable oscillation in a continuous current mode cannot be performed without varying a slope compensation value according to the voltage of the AC power source, thus requiring a complicated circuit.
In the control system of the '183 reference, the switching frequency varies with a phase θ of the AC power source. In a continuous current mode, the following equations (1) and (2) holds among an effective value Vac of the AC power source voltage, a DC output voltage Vo, an on-period of time T-on of the switching element, an off-period of time T-off of the switching element, because an increment of the inductor current in the on-period T-on equals an decrement of the inductor current in the off-period T-off in the continuous current mode in which the inductor current is continually positive or zero, supposing variation in the input voltage Vin and the output voltage Vo in one switching cycle be negligible. Note that the expression √2 in the following description means a square root of 2.T-on T-in=T-off (Vo −Vin)  (1)Vin=√2 Vac sin θ  (2)A switching frequency fs is given by the following formula (3)fs=1/T-on+T-off)  (3)Substituting the equations (1) and (2) into the equation (3), the switching frequency fs is given by the following equation (4).fs=(√2 Vac sin θ/Vo)/T-off  (4)As can be seen from the equation (4), fs is minimum at the phase θ of zero degree and maximum at the phase of 90 degrees. Since the off period T-off is fixed, the maximum switching frequency increases at high AC power source voltage, raising fear of decrease in a conversion efficiency. In order to enhance the conversion efficiency, it is desirable that the maximum frequency does not change irrespective of the AC power source voltage (regardless of a 100 V system or a 200 V system, for example). The control method of the '183 reference, however, cannot accomplish this.
The '248 reference does not mention any means for maintaining the maximum frequency in both AC power source voltages of 100 V and 200 V systems, and even in a changed AC power source voltage due to any trouble. The '248 reference is yet silent about the necessity for detecting the peak value and the average value of the supplied AC power in order to maintain the maximum frequency.